Problem: The equation of a circle $C$ is $x^2+(y-3)^{2} = 9$. What are its center $(h, k)$ and its radius $r$ ?
Answer: The equation of a circle with center $(h, k)$ and radius $r$ is $(x - h)^2 + (y - k)^2 = r^2$ We can rewrite the given equation as $(x - 0)^2 + (y - 3)^2 = 3^2$ Thus, $(h, k) = (0, 3)$ and $r = 3$.